1. Technical Field
The present invention relates to a backsheet for a solar cell which is to be disposed on a side which is opposite to a side of a photovoltaic element to which solar light incidents, a method for producing the backsheet, and a solar cell module.
2. Background Art
Solar cells are recently rapidly become popular because the cells do not emit carbon dioxide during the generation of electricity and are more environment-friendly.
Solar cell modules usually include a photovoltaic element (cell) disposed between a top surface glass, on which solar light is incident and a solar cell backsheet (hereinafter simply referred to as “backsheet”) on the opposite side from the side on which solar light is incident (on the backside). The solar cell modules include an encapsulant such as an ethylene-vinyl acetate (EVA) resin in a space between the top surface glass and the photovoltaic element (cell) and a space between the photovoltaic element (cell) and the backsheet.
The backsheet acts to prevent intrusion of moisture from the backside of a solar cell module. Although glass, fluorinated resin, and the like have been conventionally used for the backsheet, polyester is increasingly used in view of cost reduction.
In recent years, attempts have been made to raise the driving voltage of a solar cell module in order to reduce a size of a solar cell module apparatus and improve its efficiency.
Thus, there is an increasing demand for a solar cell backsheet to have a high dielectric strength in terms of safety. In recent years, a backsheet has been requested to have a dielectric strength of 600 V or more and further 1000 V or more. Correspondingly, a backsheet has been requested to have a thickness of 125 μm or more and further 250 μm or more.
A backsheet may be more than a mere polymer sheet and may be provided with various functions. For example, it may be requested to provide a backsheet which has a reflective performance by having a functional layer which includes white inorganic particles such as titanium oxide and is disposed on its polymer sheet (support) to. Such backsheet improves efficiency of power generation by diffusely reflecting (hereinafter simply referred to as “reflect”) solar light which falls on a top surface of a solar cell module and passes through a cell thereby returning the light into the cell.
A back surface protection sheet which includes a white ink layer including a white pigment is disclosed (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2006-210557).
A protection sheet for a solar cell module, the sheet including, as another functional layer, a weatherproof layer including a fluorine-containing resin is disclosed (see, for example, JP-A No. 2010-20601).
(JP-A No. 2011-165967 discloses a backsheet having a polymer substrate which includes a small amount of white inorganic particles and a reflection layer which is formed on the polymer substrate by coating and which includes a binder and white inorganic particles in a specified ratio.
International Patent Application Publication WO2009/097024 discloses a backsheet including a layer which includes fluoropolymer cured on a substrate and which further includes hydrophobic silica.